For HHG it is crucial to increase HHG conversion efficiency and to extend
the harmonics cutoff [1]}. Specifically, the conversion efficiency of
the HHG process strongly depends on the density of emitters and the density
of states of emitters. The use of molecular systems, clusters, and crystals
can significantly increase the harmonic intensity by utilizing multiple
excitation channels [2]}, [3]}, [4]}. Thus, in the last
decade, there has been a growing interest to extend HHG to crystals [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]} and two-dimensional
nanostructures, such as semimetallic graphene [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, [23]}, [24]}, [25]}, semiconductor
transition metal dichalcogenides [26]}, [27]}, and dielectric hexagonal
boron nitride [28]}. Currently, this is a new growing research field –
extreme nonlinear optics of nanostructured materials.
